Process optimization for dilute acid and enzymatic hydrolysis of waste wheat bread and its effect on aflatoxin fate and ethanol production

Abstract

Waste wheat bread was hydrolyzed using amylolytic enzymes and dilute hydrochloric acid, and the effects of process variables on glucose yield, aflatoxin fate, and ethanol production were studied. Dilute acid hydrolysis was performed with a full factorial experimental design with three levels for acid concentration, i.e., 0.32%, 1%, and 2% (v/v), and three levels for hydrolysis time, i.e., 0, 10, and 20 min at 121 °C. For enzymatic hydrolysis, the samples were first liquefied, and the saccharification of liquefied samples was performed via a rotatable central composite design with two levels of 10 and 48 h for time and two levels of 100 and 150 g/L for substrate loading. The highest glucose yield of 69.8% was obtained from the experiment with acid concentration of 1% and hydrolysis time of 20 min. The maximum glucose yield of 93% was obtained in enzymatic hydrolysis at 55 h and 125 g/L substrate loading. The general linear model was used to model the glucose yields obtained in dilute acid hydrolysis, while the results of enzymatic hydrolysis were best fit to the quadratic model. In the optimized condition for dilute acid and enzymatic hydrolysis, aflatoxin B1 was completely removed by dilute acid hydrolysis, and amylolytic enzymes enabled to remove up to 75% aflatoxin B1. The highest glucose obtained during dilute acid and enzymatic hydrolysis yielded a maximum of 86.9% and 83.0% of theoretical ethanol, respectively, corresponding to 248 and 313 g ethanol per kilogram dry bread residues.